As one of solutions for suppressing a noise component included in a captured voice signal, there is a coherence filtering. Japanese patent laid-open publication No. 2008-70878 discloses a coherence filtering method, in which the cross-correlation function of a signal being null in its right and left is multiplied for each frequency, so as to suppress noise components unevenly distributed in its arrival direction.
The coherence filtering is effective at suppressing noise components, but may cause an allophone component, i.e. musical noise, a sort of tonal noise.